Electric discharge device



Oct. 19, 1948. D. L. SMART 2,451,938

' ELECTRIC DISCHARGE DEVICE Filed April 2, 1947 WITNESSES: lNVENTOR D0 v/dL.5mar/T A M JAZ 5M ATTORNEY don, England, a"'B'ritish company Ilcrimer Smart, staffdrm l ingland, assi utu toThe English'Electric Co'mpa'nyLimitedi Lon- Application April 2, 1947, Serial No. 738,978 In Great Britain November 24, 1945 Section 1, Public Law 690, August 8, 1946 8 Claims. 1

My invention relates to an electric discharge device and particularly to a device of the kind having a liquid pool cathode and having an ignition electrode separated by a dielectric from an edge of the free surface of the cathode pool.

In such an arrangement, the ignition electrode may take the form of an external electrode embracing a vessel of dielectric material which contains the cathode pool. In an alternative such arrangement, the ignition electrode is surrounded by a tube of dielectric material and this tube dips into the cathode pool.

According to the present invention, the dielectric which separates the ignition electrode from the pool edge comprises at least two layers of different materials, the layer in contact with the pool edge being thinner than, and having lower specific inductive capacity than, the next layer.

Other features of the invention will appear from the following detailed description taken in conjunction with the accompanying drawing, in which the figure is a sectional elevation of a portion of a metal cased mercury vapor rectifier showing the ignition arrangement according to the invention.

The mercury pool cathode I2 rests upon the bottom l3 of the discharge vessel. The igniter comprises a tube l of rutile ceramic. This tube I is closed at the bottom end and is covered with a thin layer of glaze 2 over the whole of its exterior surface. A clip 3 of conductive material supports the tube so that it dips into the mercury pool cathode, and this clip is mounted on a conducting support rod 4 which enters the rectifier vessel through an insulating vacuum seal 5. A layer 8 of platinum or other conducting material is deposited on the interior surface of the tube I, and a lead 1 connects this layer electrically to clip 3. The whole of this assembly may be covered by a shield 6 substantially at cathode potential to prevent breakdown between the ignition electrode and other parts of the discharge device. A ring 9 of molybdenum or other suitable material serves to keep the mercury surface around the tube l relatively free from undesirable oxides or other material floating on the mercury. The rod 4 is connected through a resistance It! and wire I I to a firing circuit which is arranged to supply suitable ignition voltages. After ignition has taken place, the resistance l serves to control the fiow of current from the capacity including the self capacity of the wire II and the firing circuit, so that this flow of current may maintain a discharge to the cathode from the ignition electrode and its leads 4, 3, l for a sumcient time to enable a discharge to be established from a main or another auxiliary electrode in the rectifier. Resistance I0 also prevents the establishment of oscillations of potential on the ignition electrode, or causes rapid clamping thereof, and thereby minimizes the periods during which the ignition electrode is subject to high electric stress.

In the illustrative embodiment of the invention, the specific inductive capacity of the rutile ceramic, of which tube l is composed, is about 100. That of the glaze is about 5. The wall thickness of tube l is 0.02" and the thickness of the glaze is 0.002". With these dimensions, an ignition voltage of 3 kilovolts is used. In the event of a puncture at any point of the glaze 2, the resulting electrical stress on the ceramic tube I under the same ignition voltage will be insufiicient to cause breakdown.

As alternatives to rutile ceramic, which is composed mainly of rutile, a ceramic composed mainly of other forms of titanium oxide may be used, or a ceramic composed mainly of magnesium titanate. Other ceramics may be used but they should preferably be of high specific inductive capacity.

In other embodiments of the invention, additional layers of dielectric inside the tube l are employed, either for greater mechanical strength or to decrease the danger of puncture by electrical breakdown.

I claim as my invention:

1. An electric discharge device having an ignition electrode separated from a free edge of a liquid pool cathode by a dielectric comprising a plurality of layers of different materials, the layer in contact with the pool edge being thinner than, and having lower specific inductive capacity than, the next layer.

2. A device according to claim 1, wherein the dielectric comprises two layers only.

3. A device according to claim 1, wherein the layer in contact with the pool edge consists of glass.

4. An electric discharge device having an ignition electrode separated from a free edge of a liquid pool cathode by a dielectric comprising a plurality of layers of different materials, the layer of material second from the pool edge consisting of ceramic and the layer of material in contact with the pool edge being thinner than and having lower specific inductive capacity than said ceramic second layer.

5. A device according to claim 4, wherein the ceramic is composed mainly of titanium oxide.

6. A device according to claim 4, wherein the ceramic is composed mainly of rutile.

7. A device according to claim 4, wherein the,- ceramic is composed mainly of magnesium titanate 1 7, I I

In c actai i s a de ice; 1 a i liquid pool cathode, an ignition electrode in con-;

tact with the cathode pool comprising a body 99 3 conducting material dipping into the pool, a

ductive capacity of about lgQi anga h u about 0.02 inch and a layer of vitreous 4 on said layer of ceramic material, said layer of vitreous material having a specific inductive capacity of about 5 and a thickness of about 0.002 inch.

. mm aoalm M REFERENCES CITED gThe following references are of record in the aleeithisna layer of ceramic material on said-body,---said r107 layer of ceramic materialha y ing a UNITED STATES PATENTS Iggme Date -,$H,15381 June 7, 1944 

